Light olefins such as ethylene, propylene, etc. are the basic raw material for chemical industry. Conventionally, ethylene and propylene mainly come from steam cracking of hydrocarbon feedstock such as naphtha, light diesel oil, hydrogenation cracking tail oil and the like. Recently, as the price of crude oil has been dramatically rising up, the cost for producing ethylene and propylene from the above feedstock has been increasing. Also, in the conventional processes for producing ethylene and propylene, high temperature tubular furnace cracking technologies with a high energy consumption are generally used. All these factors urge the development of new olefin production technologies. Novel technical pathways of preparation of lower olefins from non-petroleum materials have caught much attention in the past years. Among those, the one, characterized in that coal or natural gas is transformed into methanol through syngas, and then methanol is transformed into lower olefins, has been of extensive interest. The process of selectively producing light olefins from methanol (or dimethyl ether generated from the dehydration of methanol) in the presence of molecular sieve catalysts is referred to as MTO process. The methods and technologies for transforming methanol into light olefins are closely correlated to the catalysts used therein. It is well known that two classes of catalysts are used in these processes. One class is the catalysts based on ZSM-5 molecular sieves with medium size micropores, characterized by higher yield of propylene and lower yield of ethylene in the distribution of the products, slightly lower total yield of ethylene and propylene, strong anti-coking ability of the catalysts and longer operating cycle. The reaction techniques suitable for the above class are usually fixed bed reaction techniques with periodical switches between reactions and regenerations. Another class of catalysts is based on the molecular sieves with smaller pore size, characterized by high total yield of ethylene and propylene as well as higher yield of ethylene in the product. Because this class of catalysts has a higher coking rate, the processes generally utilize fluidized bed techniques with continuous reaction-regeneration of the catalysts.
U.S. Pat. No. 6,613,951 B1 and Chinese patent CN1352627A disclose methods for converting methanol or/and dimethyl ether into C2-C4 olefins, wherein the feed are contacted with a catalyst containing a 10-ring zeolite under 370-480° C. with a methanol partial pressure of 30-150 psia.
Chinese patent CN1302283A discloses a method for converting methanol or/and dimethyl ether into C2-C4 olefins and aromatics higher than C9, wherein a portion of the aromatics is returned back to the reactor and co-fed with methanol or dimethyl ether under 350-480° C. to increase the yield of olefins. U.S. Pat. No. 6,506,954 B1 discloses a method for converting methanol or/and dimethyl ether into C2-C4 olefins, wherein aromatic compounds are added to increase the yield of olefin. The catalysts used in the method are porous crystalline materials with a pore size larger than the dynamic diameter of aromatic compounds. Reactions are conducted at 350-480° C. The partial pressure for methanol is not lower than 10 psia. The 2,2-dimethyl butane diffusion coefficient of the catalysts is 0.1-20 sec−1 (120° C., 60 torr). U.S. Pat. No. 6,538,167 B1 discloses a similar method, suggesting that the reaction conditions shall ensure the alkylation of aromatic compounds.
U.S. Pat. No. 6,710,218 B1 discloses a method for converting methanol into lower olefins, utilizing SAPO-34 as the catalyst. A fluidized bed reactor-regenerator process is used. The selectivity of lower olefins is higher than 90 wt %, in which more than 80 wt % is ethylene and propylene. The ethylene/propylene ratio ranges from 0.69 to 1.36 by the modification of the reaction temperature and space velocity of feed.
U.S. Pat. Nos. 6,437,208 B1 and 6,740,790 B2 disclose a method for making olefins from oxygenate-containing feedstock by employing a fluidized bed reaction technique in the presence of a silicoalumophosphate molecular sieve catalyst. The conversion conditions include that the silicon/aluminum ratio in the catalyst is lower than 0.65; the average catalyst feedstock exposure index (ACFE) is at least 1; and the reaction temperature is 200-700° C., etc.
U.S. Pat. No. 6,455,747 B1 discloses a method for converting oxygenate-containing feed into olefins. The reaction conditions include that the superficial velocity is not lower than 2 m/s and WHSV is 1-5000 hr−1. Similar methods are disclosed in U.S. Pat. Nos. 6,552,240 B1 and 6,717,023 B2.
Chinese patent CN1163458C discloses a MTO reaction process using SAPO-34 catalyst and a dense phase fluidized bed. The conversion of methanol is 93-100%. The total selectivity for ethylene and propylene is higher than 80% by weight. The ethylene/propylene ratio of the product may be changed by altering the conditions such as reaction temperature, feeding space velocity and the like.
U.S. Pat. No. 6,613,950 B1 discloses a method for producing olefin from oxygenate-containing feedstock. A silicoalumophosphate molecular sieve is utilized as the catalyst. Following gas stripping, a portion of the exposed catalyst is returned back to the reaction zone without regeneration to be contacted with the feedstock.
U.S. Pat. No. 6,743,747 B1 discloses a method for converting oxygenate feedstock into olefins with a silicoalumophosphate molecular sieve as the catalyst. Aromatic compounds are added to the feed proportionally, so that the yield of olefins, in particular ethylene, is enhanced.
U.S. Pat. No. 6,051,746 A1 discloses a method for converting oxygenate organic material into olefins with small pore molecular sieve catalysts. The catalyst is pre-treated with the aromatic compounds containing nitrogen and at least 3 interconnected rings, in order to decrease the amount of byproducts such as ethane, etc. and increase the yield of olefins.
U.S. Pat. No. 6,518,475 B2 discloses a method for converting oxygenates into lower olefins. The catalyst utilized is a silicoalumophosphate molecular sieve. In order to obtain a higher yield of ethylene, acetone is added to the feed, or the catalyst is pre-treated with acetone.
US patents US20050215840 A1 and U.S. Pat. No. 6,965,057 B2 discloses a method for converting oxygenate feedstock, including methanol, into lower olefins. A riser technique is used. At least a portion of the catalyst deactivated by coking enters the regenerator to burn off the carbonaceous deposit. At least 60% of the molecular oxygen carried by catalyst is then removed by gas stripping. The catalyst is returned back into the reactor to be re-contacted with the feedstock.
U.S. Pat. No. 6,673,978 B2 discloses a method for converting oxygenate feedstock into olefins A silicoalumophosphate molecular sieve is used as the catalyst. The fluidized bed reaction device employed includes at least one reaction zone and one circulation zone. A temperature of 250° C. or higher is set up for the circulation zone, and specified portions of the catalyst circulates between the reaction zone and circulation zone.
Chinese patent CN1356299A discloses a technique utilizing SAPO-34 catalyst and a parallel-flow descending type fluidized bed. With this method, the byproduct such as alkanes, etc. in the MTO procedure is decreased, thus the difficulty of the subsequent separation is lowered. The conversion of methanol is higher than 98%, and the selectivity of olefins is higher than 90 wt %.
Chinese patent CN1190395C discloses a method for producing lower olefins from oxygenate compounds, such as methanol or dimethyl ether, comprising the step of feeding at several locations along the axis direction of the catalyst bed, which improves the selectivity of ethylene.
Chinese patent CN1197835C provides a method for transforming oxygenate compounds into olefins. A main olefin product yield of 45 wt % could be achieved under the conditions that the feeding is carried out while the oxygenate proportion index is at least 0.5; and the partial pressure-rate compensation factor is kept at least 0.1 psia−1hr−1.
With the technologies and methods disclosed above, the distributions of products are relatively fixed. Although the composition of products could be modulated by altering the reaction conditions in some of these patented methods, the extent of the modulation is much limited. The changing demand for olefins in the global market, particularly the rapidly increasing demand for propylene in the recent years, requires a more flexible distribution of the olefin products transformed from methanol, especially in the ratio of ethylene to propylene, i.e. the two main olefin products. In view of above, several patents have disclosed the methods for changing the distribution of products by recycling a portion of the products.
U.S. Pat. No. 6,441,262 B1 and Chinese patent CN1489563A disclose methods for transforming oxygenate feedstock into lower olefins, wherein methanol, ethanol, 1-propanol, 1-butanol or mixture thereof is contacted with the catalyst to generate olefins, and then the catalyst is contacted with said oxygenate compounds to generate olefins, so that the ratio of products, including ethylene, propylene and butylene, could be changed without shutdown.
Canadian patent CA2408590 discloses a technique for transforming methanol into propylene. Reactors connected in series are utilized to produce propylene, dimethyl ether and higher hydrocarbon, and the resultant dimethyl ether and a portion of the higher hydrocarbons are returned back into the reactors connected in series and subjected to a further reaction, in order to increase the yield of propylene.
U.S. Pat. Nos. 5,914,438 and 6,303,839 B1 disclose methods for producing lower olefins, including that oxygenate feedstock are contacted with the catalyst containing aluminophosphate and transformed into C2-C4 olefin, and partial C3 and C4 fraction are recycled and cracked to increase the yield of ethylene and propylene. The recycling reaction could be carried out in the riser of the fluidized bed or in a separate reaction zone.
U.S. Pat. No. 5,990,369 disclosed a method for producing lower olefins, including that oxygenate feedstock are contacted with a catalyst containing aluminophosphate and transformed into C2-C4 olefins, and a portion of the olefin products are recycled and cracked to increase the yield of ethylene, propylene and butylenes, wherein either propylene could be recycled to increase production of ethylene, or the ethylene and butylenes could be recycled to increase production of propylene.
The present invention is based on the theory that during the transformation from methanol to olefins, there exist several reaction trends: Certain reaction trends can be enforced by employing different reaction conditions in different reaction zones, so that the composition of the products can be changed. The commonly well known transformation route from methanol to hydrocarbons under acidic catalysts is shown as follows:

It is a complicated reaction network, roughly including two reaction directions: one is the reactions involving the increase of the number of carbon atoms—light olefins such as ethylene, propylene and the like forms higher hydrocarbon molecules through reactions such as oligomerization and the like; the other one is the reactions involving the reduce of number of carbon atoms—higher hydrocarbon molecules are cracked into light olefin such as ethylene and propylene, etc. In order to obtain lower olefins such as ethylene and propylene, certain reaction conditions such as higher reaction temperature which favors the transformation towards the cracking of higher hydrocarbon are usually required. Another pathway is to utilize the shape selectivity of molecular sieve catalysts to ensure that the reactions occur in the channels so that only the smaller hydrocarbon molecules may diffuse out and lower olefins are formed at a high selectivity. Recently, it has been found in some studies that the reaction types involved in the MTO transformation procedure are more complicated than the above scheme. For instance, Svelle et al has discovered in an isotope research (Kinetic studies of zeolite-catalyzed methylation reactions. J. Catal., 224 (1), 115-123, 2004) that during the transformation from methanol to olefin, alkylation occurs between olefins and methanol, so that the number of carbon atoms in olefin is increased:CH3OH+CnH2n=Cn+1H2n+2+H2O
In particular, the alkylation between one molecule of ethylene and one molecule of methanol forms one molecule of propylene.
U.S. Pat. No. 3,906,054 discloses a technology for alkylation of olefins, comprising that olefins are contacted with catalysts, i.e. a zeolite with silica to alumina ratio of 12 or higher, in the presence of alkylating agents. P modification is used, wherein the minimum P content is 0.78 wt %. The olefins that may be alkylated include ethylene, propylene, butylene-2 and isobutylene, and the suitable alkylating agents are methanol, dimethyl ether and methyl chloride.
The international patent WO2005/056504 A1 discloses a method for efficiently preparing propylene from ethylene and methanol or/and dimethyl ether, including that ethylene and methanol or/and dimethyl ether react in the presence of catalysts to generate propylene. It is characterized in that the amount of ethylene flowing out of the reaction system is reduced as compared with the amount of ethylene added to the reaction system. The yield of propylene reaches 40 mol % based on the moles of methanol or two times of the moles of dimethyl ether which is added to the reaction system.